Elastomer foam containing colloidal silica



ELASTONIER FOAM CONTAINING COLLDIDAL SILICA Ralph K. ller, Brandywine Hundred, DeL, assignor to 'E. I. du Pont de Nemours and Company, Wilmington,

Del., a corporation of Delaware NoDrawing. Original application August 1, 1951, Serial No. 239,853. Divided and this application September 9, 1953, Serial No. 379,292 7 4 Claims. (Cl. 260-) an. average ultimate particle diameter of about 0.01 to 0.03 micron.

Water-soluble alkali metal silicates having calculated weight ratios of SiOzzalkali (in which the alkali is calculated as NazO) up to 4:1 are well known commercial products, and procedures for making colloidal silica solutions from such silicates as, for instance, by acidifying them, are also well known. Unfortunately most of the colloidal silica solutions made in this manner are unstable, precipitating as gels within relatively short times after preparation. Accordingly, it has long been considered impracticable to make and transport colloidal silica solutions in the usual commercial channels.

Recently, however, compositions which may be described as alkali-stabilized colloidal silica solutions have become available. These colloidal solutions, the preparation of which is described, for instance, in Bird United States Patent 2,244,325, Bechtold andSnyder U. S. Patent 2,574,902, and Rule U. S. Patent 2,577,485 are stable against gelling over prolonged periods, so that it is err-- tirely practicable to transport them and use them at times and places considerably removed from their point of preparation. They contain an alkali such as the hydroxide of sodium, potassium, lithium, or ammonium,"

or an organic amine. The weight ratio of SiOz to alkali 10:1 up to as high as 500:1, making it obvious that the alkali and silica are combined in a special manner not found in conventional alkali metal silicates, since thelatter cannot be prepared in a form soluble and stable The silica in aqueous solution at ratios above about 4: 1. in the sols may have an average particle diameter of about .01 to .03 micron.

The availability of silica in the form of stable sols as above describedhas, according to the-present invention, made possible novel and useful improvements in organic polymeric compositions. It has previously been proposed to add fine powders, including finely divided silica, to various organic polymers, but the silica powders heretoforetried have suffered from numerous disadvantages, often attributable to the size and character of the silica added and its method of incorporation into the polymers. For instance, it is well known that finely divided powders, such .as carbon blacks, impart valuable properties to rubber and rubbery materials. More recently it has been proposed to use finely divided silica for this purpose, and in U. S.,Patent 2,399,687, for instance, the preparation of silica in a finely divided form said to be particularly adapted for use as a filler and reinforcing agent in rubber has been described. The methods hitherto suggested for incorporating fine silica into rubber have been similar to,

those used for incorporating carbon blacks and have 2,760,941 Patented Aug. 28, 19 56 "ice required milling the rubber and silica together, as on compounding rolls. By such methods the silica added is dispersed but any hard aggregates of silica are not broken down, and it has been considered that breaking down such hard aggregates on the rubber mill was not possible.

Moreover, from a consideration of the physical nature of the silica powders which it has heretofore been proposed to add to elastomers it has not been apparent that such powders could be incorporated in any manner other than milling them into the elastomer.

It has heretofore been proposed to modify plastics by the inclusion of silica in finely divided form. Silica ordinarily cheapens a plastic but does not lend properties of any great value.

Silica in the form of finely ground particles down to a size no smaller than about, say, 10 or 15 microns acts in a plastic as an inert filler. Tensile strength is reduced because the bond between the plastic and the silica is weaker than the cohesive forces of the plastic itself and the filler therefore weakens the finished product. Theoretically, silica gel in finely divided form should pre sent a large surface to a plastic and it might be expected that the tensile strength would be greatly increased because of the large surface forces alforded by the gel structure. The silica gel does not elfectively present the surface because the plastic will not penetrate the micropores. The gel, moreover, is not easily broken down into fine particles to take advantage of its structure. The gel cannot be completely broken to a small particle size within the colloidal range, but breaks down into angular,

, irregularly shaped particles many of which are relatively found that by introducing silica, as an alkali-stabilized sol of ultimate particles .01 to .03 micron in diameter, into organic polymers having a molecular weight of more than 10,000, there are provided novel and highly useful compositions comprising solid dispersions containing the silica in a matrix of the polymer. In preferred aspects of the invention, the sol used is prepared by a process including the step of passing an alkali silicate solution through an acid-regenerated ion-exchange resin, and the silica particles in the sol are dense and substantially spherical and discrete.

THE SILICA SOL USED The alkali-stabilized colloidal silica used in a composition of this invention is characterized by containing silica in the form of particles of colloidal dimensions. Although the colloidal range includes particles having an average diameter not exceeding about 0.1 micron nor less than about 0.001 micron, it is preferred in this invention to use colloidal silica having an ultimate particle size in the range from 0.01 to 0.03 micron, that is, 10 to 30- millimicrons, By ultimate particle size is meant the average diameter of particle present as measured with an electron microscope when the solution is diluted to about 0.1% SiOz or less with water and dried in a very thin layer deposit.

Particles smaller than about 10 millimicrons, while effective in regard to their reinforcing action in organic polymers, are not as dispersible as particles larger than 10 millimicrons, when used in the form of an aqueous sol, because such sols show a strong tendency to gel and the ultimate particles aggregate very readily. It would be possible to use sols in which the average particle size is smaller than 10 millimicrous if the silica concentration was kept very low, but this introduces the ditliculty that when the dilute sol is mixed with the organic polymer system, the whole mixture is diluted to an undesirable extent. For greatest utility, the silica concentration in the sol to be used should be as high as possible, and it is generally found that more concentrated sols can be prepared in'a stable state only when the particles are larger than about 10 millimicrons, in fact preferably larger than about 15 millirnicrons.

On the other hand, it is preferred that the average Pa c i in h o be les th n 30 millimi ron i iem erximum rei f ing efiec s and other benefi d ri ed t o dm x u o colloidal silica wi h organic polymers are obtained when the particles are as ma as p ss b e It is a so advantageous to ha e a 1 in whichthe ultimate particles are individual, discrete, spherical units. particles in the size range of to 3.0 millimicrons are not aggregated to form clusters, 1 reinfOreing effect, when the sol is mixed with a dispersion of an organic polymer and the mixture is coagulated, is greater th n it would be if the sol particles were aggregated.

The silica particles are not present as anhydrous silicon dioxide, but rather, are in a hydrated f m a sociated with various proportions of Water. Thus, partially dehydrated silicic acid would come within the term isilica as herein used. The silica particles are amorphous, as indicated by their X-ray diffraction patterns.

The silica sols are characterized by having hydroxyl ions and the corresponding cations of the alkali so disposed With relation to the silica particles as to prevent condensation between the particles by oxolation in aqueous solution at a concentration not exceeding about 45% SiOz by weight. As freshly prepared, the silicic acid in a silica solution may exist as low molecular weight polymers of the theoretical Si(OH)4; however, a condensation reaction known as oxolation rapidly occurs whereby water splits out between hydroxyl groups attached to silicon atoms, an Si-O-Si linkage is, formed, and a polymer is produced. This process may continue indefinitely and,

in such a manner that the silicic acid solution rapidly sets up to a gel, the .oxolation occurring between externally disposed hydroxyls, i. e., hydroxyls not already inactivated by being inwardly located within a siloxane ring or micelle. It has been found, however, that such polymers.

completely inadequate to render soluble a conventional silicate glass or to peptize it to form a stable colloidal solution.

The Colloidal silica solutions used in making the com positions of the present invention are characterized by containing silica in the form of discrete ultimate particles, and the alkali present is not uniformly distributed throughout the particles, but rather, is substantially all outside of the particles, except perhaps for occluded traces. Being external, the cations of the alkali can be removed by such methods as dialysis or ion-exchange using an acid-regenerated cation exchange resin, without afiecting the structure of the silica particles. The resultant alkali-free silica sols are unstable toward gelation, but they may be restabilized by adding back a small propor-. tion of an alkali such as sodium hydroxide. Hence, the

colloidal silica solutions used according to this invention Since, in the 5.01, the individual silica g 60:1 give superior results and are preferred,

may properly be referred to as alkali-stabilized." This stabilization may be accomplished by processes described in the Bird Patent 2,244,325, In order to produce stable solutions with a minimum of alkali stabilizing agent, the polymerization of the silicic acid may be carried to a relatively high molecular weight so as to produce large molecules or discrete colloidal particles which then retain only a small percentage of the reactive hydroxyl groups originally present, and which thus require only a small amoun f alkali as a stabilizer. On the other han the polymerization must not be allowed .to proceed in such a way as to form a gel through the aggregation of Such a gelation would the original colloidal particles. occur, for instance, if insufficient alkali were present.

A practical method for producing stabilized aqueous colloidal silica solutions consists in passing an alkali silicate solution through an acid-regenerated ion-exchange resin as described in the above-mentioned Bird patent whereby alkali ions are removed irom the silicate and replaced with hydrogen ions. The alkali ions may be completely removed and thenelkali hydroxide may be adde o furnish the necessary ions required for stabilization, or a portion of the original alkali ions in the silicate may be allowed to remain in the solution as stabilizing ions. In either case, upon subsequent evaporation of the water to concentrate the sol, particularly at temperatures in the neighborhood of the boiling point at atmospheric pressure, growth of the silica particles occurs, and this may be continued until the particle size is at least 1.0 millimicron The alkali-stabilized colloidal silica solutions incorporatedlinto the organic polymer compositions of this invention are still further characterized by having, in dilute aqueous solution, an extinction coeflicient of less than 0,25 for light having a wavelength of 400 milli microns. A discussion of the method for determining the extinction coefi cient is given below under Characterization techniques.

A particular stabilized aqueous colloidal silica solution which may be used, in a composition of this invention is described in theBird patent at page 2, column 1, lines 12 to 68. As stated in the Bird patent the weight ratio f SiO2 t Naeo may be as highes fr m to :1 and as lowas 10:1 In the present invention ratios above The higher the ratio, of course, the lower will be the proportion of cations from the alkali.

Metho s other than the one. ab ve described for prouciusst bilizcd colloidal silica solutions from alkali ilicates by ion-exchange methods may be employed. The use of cation cxchangers in various relations is gen? erally. well understood and is described, for instance, in

the aboyc mentioncd Bird Patent 2,244,325 and in the of sulfonated carbonaceous exchangers or of sulfonated or sulfited insoluble phenol-formaldehyde resins or acidtreated humic material, or other similar exchangers being typical.

Even more preferable are the insoluble resins made from phenols and an aldehyde, particularly formaldehyde.

Such resins include those made from phenol itself, di-

phenylolsulfone, catechol, or naturally occurring phenols,

s found, for example, in oueb'racho, which are modifie y he introduction otsul fonic groups either on he ring. r on methylene groups. Cation-exchangers which are.

able in their hydrogen forms are available commercially nder such trade names as Amberli e, Zcokarbf Nalcitef. and ;Ionac. ffAniberlite 1s .a mod1fied Sulfonated coal, lignin, poet, or other insoluble .su lfonated humic organic material may be used.

phenol-formaldehyde-sulfonic acid-type resin, Zeokarb is a sulfonated coal of the carbonaceous zeolite type, Nalcite is a nuclear sulfonated polymer of styrene containing divinyl benzene, and Ionac is a phenol-formaldehyde sulfonatesee Ion-Exchange Theory and Application by F. C. Nachod, Academic Press, Inc., New York, N. Y., 1949, at page 385 et seq.

If it is desired to replace, say, sodium ions with hydrogen ions, the exchanger should be initially in the acid form, Whereas, if it is desired to replace the sodium ions with ammonium ions the resin should be converted to the ammonium form as shown in Voorhees 2,457,971. It will be understood that to regenerate a spent exchanger to the acid form, even moderately weak acids will often be sufiicient.

The exchanger is generally prepared in a granular form which is readily leached free of soluble acids or salts. If the exchanger is exhausted by use it may readily be converted to the acid form by washing with a solution of an acid such as hydrochloric, sulfuric, sulfamic, or a carboxylic acid such as formic, or the like. The exchanger may be treated instead with an ammonium compound such as ammonium hydroxide or ammonium chloride.

One of the preferred cation-exchange resins for use according to the present invention is an aromatic hydrocarbon polymer containing nuclear sulfonic acid groups which is designated Dowex 50 and of the general type described in DAlelio 2,366,007 and which is fully described as to its characteristics, properties, and general mode of use in the Journal of the American Chemical Society for November 1947, volume 69, No. 11 beginning at page 2830.

The disclosure in the Bird patent describes the preparation of an effluent from the ion-exchanger having a silica content of about 3 to 3.5% which may be evaporated, preferably in a vacuum evaporator to about one-half its original bulk thereby raising its silica content to about 6 to 6.5%, the resulting sol being relatively stable. A silica sol having a 15% silica content is mentioned. Even at these concentrations the solutions are too dilute to be shipped, stored, and handled economically. It would therefore be preferable to evaporate them to an SiOz content of from 18 to 30%. This may be done in a number of ways and the product obtained will have somewhat different characteristics depending upon the method of concentration.

For instance, particularly stable, useful sols may be prepared by recently discovered concentration techniques involving particle build-up or growth, as described in the above-mentioned Bechtold .and Snyder patent. In this case the concentration may be carried out at elevated temperaturethat is, above about 60 C. This may be done in the batchwise manner at constant volume, by adding a suitable volume of filuent from the ion-exchanger to the evaporator and adding additional fresh efiluent at the same rate that Water is evaporated oif so that the volume in the evaporator remains substantially constant. Preferably, the process Will be continued until at least five times as much fresh effluent has been added as was originally present. The evaporation may be carried out at atmospheric pressure or even superatmospheric pressure to give a product having maximum stability and containing as high as 30 to 35% SiOz in the form of dense, discrete particles. The process may readily be controlled so that the molecular weight of the silica is greater than 0.5 million as determined by light scattering techniques described hereinafter and the average particle size may be below 30 millimicrons as determined from electron micrographs. The silicazalkali ratio is in the range of from 60:1 to 130:1.

-An alterative method of evaporation, which gives a product of higher molecular weight but still of colloidal dimensions, may be obtained by carrying out the evaporation at constant volume but in a continuous manner. Thus, from .a constant volume in the evaporator concentrated product is continuously withdrawn and fresh eflluent is added at a rate sufficient to maintain constant volume in the evaporator and to make up the decrease efiected by evaporationand product Withdrawal. The product thus obtained contains particles of substantially uniform size corresponding to a molecular weight lying in the range of about from 0.6 to 100.0 million, but, of course, only such products having a particle diameter less than 30 millimicrons are employed in this invention.

The Bechtold and Snyder sols are characterized in general by being stable, aqueous silica sols, by having a silicazalkali mole ratio of from 60:1 to 130:1 (or equivalently, a weight ratio within this range where the alkali is expressed in terms of the Weight of NazO equivalent to it), and by containing non-agglomerated silica particles having a molecular weight of more than A million as determined by light-scattering, the sols having arelative viscosity of from 1.15 to 1.55, at 10% SiOz and a pH of 10 at 25 C., and containing substantially spherical, non-agglomerated, dense, ultimate particles, substantially all of which have an average diameter in the range of 15 to 130 millimicrons and at least of which have an average diameter of from 0.5 to 1.7 times the arithmetic mean particle diameter. The sols contain an alkali such as the hydroxide of lithium, sodium, potassium, rubidium, cesium, ammonia, or an organic amine. The silica concentration is from 20 to 35% by weight of SiOz. The silica is amorphous, as determined by X-ray diffraction.

Particularly preferred for use in this invention are colloidal silica sols which may be prepared according to the process of Bechtold and Snyder and which have the following characteristics: The aqueous sols are practically clear, having only a slight opalescence. They are stable indefinitely under ordinary conditions of storage and may be stable for many months at C. The sols have an SiOzrNazO mole ratio of from 80:1 to :1 and a pH of from 9.5 to 10.5. The sols contain substantially spherical, non-agglomerated, dense, ultimate particles of amorphous silica, substantially all of which have an average diameter in the range of from 15 to 30 millimicrons, and at least 80% of which have an average diameter in the range of 0.5 to 1.7 times the arithmetic mean particle diameter as measured by means of an electron microscope. The spherical, non-agglomerated nature of the particles is also indicated by the low viscosity of the sols which in this preferred case is an absolute vis-- cosity or" 2.0 to 4.0 centipoises as measured directly at 25 C. on a sol containing 30% SiOz at a pH of 9.7, or a relative viscosity of 1.15 to 1.55 as measured at 10% SiGz concentration and a pH of 10 at 25 C. When the silica sols are adjusted to a pH of 3.5 with dilute hydrochloric acid and evaporated to dryness, the resulting powder possesses a specific surface area as measured by nitrogen adsorption of from 80-300 M /g. The specific gravity of the sols at 60 F. is from 24.3 to 26.1 B6. The freezing point of the sols is 32 F. The silica particles have a molecular weight as determined by light scattering of the sol of more than one-half million and less than 90 million. The sols have an extinctioncoefi'icient of less than 0.24 for light of a wavelength of 400 millimicrons. The sols have the following chemical analysis:

SiOz 29 to 31%. Titratable alkalinity as NazO 0.29 to 0.39%. Sulfates as Na2SO4 0.15% Max.

The silica is amorphous as determined by X-ray diffraction.

A colloidal silica sol which may be prepared according to the process of Bechtold and Snyder and is espe cially preferred for use in this invention has the following characteristics: The sol is practically clear having only. a slight opalescenc-e. It is stable indefinitely under ordinary conditions of storage or even when kept at 95 C.. The sol has an SiOzzNazO mole ratio of 92:1 and 'a pH of 9.6to 9.8. The sol contains substantially spherical, non-agglomerated dense ultimate particles of silica, substantially all of which have an average diameter of about 15 to 20 millimicrons and at least 80% of which have an average diameter of from 0.5 to 1.7 times the Y arithmetic mean. particle diameter, as measured by means of the electron microscope. The spherical, non-agglomerated nature of the particles is also indicated by the low relative viscosity of the sol which is from 1.2 to 1.4 as measured at 10% SiOz concentration and a pH of 10 at 25 C. When the sol is adjusted to a pH of 3.5 with dilute HCl and evaporated to dryness, the resulting pow-- der possesses a specific surface area as measured by nitrogen adsorption of about 200 M /g. The silica particles have a molecular weight as determined by light scatteringot the sol of'about 3 to 8 million. The density of the sol at 60 F. is 1.2 g./cc. jThe freezing point of the sol is 32 .F. The sol has an extinction coefficient of less than 0.24 for light of a wavelength of 400 millimicrons,

' The sol will have the following average analysis:

' Per cent SiOz 30.06 NazO titratable 0.326 1 S04 as Na2SO4 0.10 Cl as NaCl 0.05 Sulfated non-siliceous ash:

Before leaching 1.20 After leaching 0.05

Sols prepared according to the disclosure of the Bechtold and Snyder patent above-mentioned may contain varying amounts of salts such as sulfates, chlorides, carbonates, bicarbonates, or the like. They also contain a considerable amount of basesince they have a silica: alkali oxide mole ratio of from 60:1 to 130:1. The content of ions in the sol prepared from commercial materials by the Bechtold and Snyder process is sufficient to make the specific conductance of the sol about X10 mho/cm. at 28 C. and SiOz. This ion content tends to decrease the stability of such sols at very high SiOz concentrations and to reduce their compatibility with organic materials.

The preparation of improved, stable, highly concentrated, substantially salt-free silica sols, which may be used in compositions of this invention, is disclosed in the Rule U. S. Patent 2,577,485. These sols are stable even when concentrated to upwards of 35% SiOz, by reason of the fact that not only do they contain amorphous silica particles which are dense, non-agglomerated, spherical and above 10 millimicrons in diameter, but that the sols are substantially salt-free and contain only enough alkali metal hydroxide to give a silica:alkali oxide mole ratio of from 130:1 to 500:1.

The sols described in the Rule patent are prepared by processes wherein a quantity of alkali metal hydroxide suflicient to produce the desired silica:alkali ratio is added to a sol containing amorphous silica particles which are dense, non-agglomerated, spherical and have an average diameter of 10 to 130 millimicrons, the sol being substantially free of electrolytes and having a relative viscosity, as measured at 10% SiOz, and pH 10, and 25 C., of 1.15 to 1.55 and a specific conductance, as measured at 10% SiOz and 28 C., of less than 4x 10' mho/cm., the amount of hydroxide added being enough to adjust the silica:alkali oxide mole ratio to from 130:1 to 500:1.

A preferred method for preparing the sols disclosed by Rule is to start with a sol of the type disclosed by Bechtold and Snyder and treat it further with ion-exchangers until it has been made substantially free of electrolytes. Thus, a Bechtold and Snyder sol may be deionized by subjecting it to successive contact with a.

' gelling, and the treatmentof such a solaccording to the cationexchanger such. as Nalcite HCR in its. hydrogen form and an anion-exchanger such as Amberlite IR 43 in its hydroxide form. The sol is comparatively unstable after complete purification by this method, particularly at higher vconcentrations-that is at concentrations above about 30% SiOz.' However, as shown in U. S. Patent No. 2,601,352 to Wolter, the sol can be stabilized by adding. a nitrogen base such as an organic amine or a quaternary ammonium hydroxide. I p

According to the method disclosed in the Rule patent, such deionized sols may be stabilized by the addition of enough alkali metal hydroxide to give a silicazalkali oxide mole ratio of from 130:1 to 500:1. The alkali metal hydroxide used may be the hydroxide of lithium, sodium, potassium, rubidium or cesium. The alkali metal hydroxide is preferably added to thesilica sol as a solution with agitation, trated to very high silica concentrations by the, direct evaporation of Water with vigorous agitation, the silica content being carried to as high as SiOz or more without gelation. For products of the highest silica content, it is preferable to use a sol containing a minimum of anions and having a silica:alkali ratio of from 150:1 to 300:1 and more particularly about 200:1.

A considerable excess of anion exchanger is ordinarily used in the Rule process and the content of anions other than OH. (e. g chloride and sulfate) drops to an exceedingly low figure and does not total more than about 0.02 to 0.03%. As pointed out above, treatment of the sol with an anion-exchange resin is preceded by treatment with a cation-exchange resin. i

It is preferred to carry out the deionization upon relatively concentrated sols-that is, above about 20% SiOz. A sol which has had the cations substantially removed at high silica concentration has some tendency toward process of the Rule application preferably is carried'out as soon as feasible after the deionization is effected.

Anion-exchangers are generally well-known. and the composition of anion-exchangers and their mode of use are fully described in the literature. Suitable materials are mentioned, for instance, in U. S. Patents 2,438,230 and 2,422,054. A description of both cation and anionexchangers will be found in the May 1945 issue of Chemical Industries in an article entitled Ion-Exchange by Sidney Sussman and Albert P. Mindler at pages 789, et seq.

While any of the anion-exchangers described will be found satisfactory, the insoluble resins obtained by the reaction of formaldehyde with an aromatic amine are par ticularly useful. Such products are described, for instance, in the U. S. Patent 2,151,883 of Adams and Holmes. Reference is made, for instance, to the metaphenylenediamine formaldehyde type of anion-exchanger in the Ryznar Patent 2,438,230 stated above. A guanidine-type anion-exchanger may also be used.

A particularly preferred anion-exchanger is common cially available under the trade name Amberlite IR-4B. This is characterized as being a modified phenol formaldehyde polyarnine condensate containing 14% nitrogen and 39.9% water in the basic form of the resin and having an exchange capacity of 10 milliequivalents per gram of resin. The properties of the resin are described further by Kunin and Meyers in the Journal of the American Chemical Society, vol. 69, p. 2874, 1947.

The stable, highly concentrated, substantially salt-free.

silica sols may also be prepared by the processes disclosed in Rule U. S. Patent 2,577,484. Such a process comprises preparing a substantially salt-free active low molecular weight silica sol by effecting contact, successively, between a sodium silicate solution and a cation-exchanger in the hydrogen form and an anion-exchanger in the hydroxide form, adding a suitable amount of alkali hydroxide to adjust the. SiO2:M2O mole ratio of the sol to from :1. to. 500:1, heating a portion of the adjusted $01 but The, resulting sol may be CUHCeDr.

a temperature above 60 C. under atmospheric pressure to'form a substantially salt-free heel containing substantially spherical non-aggregated, dense silica particles, by

adding to the heel an additional quantity of the original active silica sol while maintaining the pH in the range from 7.5 to 10.7, the temperature above 60 C. and preferably at the boiling point of the sol, whereby water is evaporated, the additional quantity of active silica sol being added at such a rate, relative to the rate of evaporation, that the volume of the combined sols remains substantially constant, the amount of silica added being at least 3 to 15 times as much as that originally present in the heel, and concentrating the sol to upwards of 35% by weight of silica by direct evaporation with agitation, the resulting sol being stable toward gelation.

The aqueous silica sols produced according to either of the above Rule patents are stable, practically clear sols containing amorphous silica in the form of dense, nonagglomerated, spherical particles having an average particle diameter of 10 to 150 millimicrons, in a concentration of from 35% to upward of 50% SiOz by weight. The sols have a silicazalkali oxide mole ratio of from 130:1

to 500:1, a relative viscosity of from 1.15 to 1.55 as measured at 10% SiO2 and a pH of 10 at 25 C., and a specific conductance, as measured at 10% SiOz, and 28 C., of less than (was) x 10- mho/cm.

' where R is the silicazalkali oxide mole ratio.

'.'A sol which may be prepared according to either of the Rule processes and is particularly preferred for use in this invention has the following characteristics: The sol is practically clear, having only a slight opalescence. It is stable indefinitely under ordinary conditions of storage and does not gel after one month of heated storage at 95 C. Electron microscopic examination shows that the sol contains substantially spherical, non-agglomerated, dense, ultimate particles of amorphous silica, substantially all of which have an average diameter of from 0.5 to 1.7 times the arithmetic mean particle diameter which is 15 to 20 millimicrons. The sol has absolute viscosity of about 250 centistokes at 25 C., and when diluted to a concentration of SiOz and adjusted to a pH of 10, the relative viscosity is 1.2 to 1.4. When such a silica sol is adjusted to pH 3.5 with dilute hydrochloric acid and evaporated to dryness at room temperature under vacuum, the resulting powder has a specific surface area as measured by nitrogen adsorption, of about 200 M g. The specific conductance of the sol which contains 40% SiOz is 2.9 10' mho/cm. at 28 C. The pH of the sol is 9.6 to 9.8. The density of the sol is about 1.30 g./cc. The freezing point of the sol is 32 F. The mole ratio It will be noted that the Bec'htold-Snyder method is capable of giving silica sols having particle sizes in the range from 15 to 130 millimicrons and the Rule sols may contain particles of 10 to 150 millimicrons in diameter. For the purposes of the present invention only sols having a particle size in the range of 10 to 30 millimicrons are employed. The particle size may be controlled by limiting the amount of build-up permitted to occur in the sols. For instance, the amount of silica added as build-up in theBechtold-Synder process may be restricted to the minimum, 5:1 build-up ratio.

" Methods of characterizing sols in the terms abov'e set forth are given below under Characterization techniques.

THE ORGANIC POLYMER The organic polymer with which silica is used according to the present invention may be any organic plastic polymer having a molecular weight greater than 10,000. The term plastic is used in its usual and ordinary meaning, as defined in Modern Plastics Encyclopedia, 1950 edition, at page 30: A plastic is any one of a large and" varied group of material which consits of, or contains as an essential ingredient, an organic substance of large molecular weight, and which, while solid in the finished state, at some stage in its manufacture has been or can be formed (cast, calendered, molded, etc.) into various shapes by flow-usually through the application, singly or together, of heat and pressure.

Included as organic polymers is that special class of substances known as elastomers. An elastomer, as defined at page 30 of the Modern PlasticsEncyclopedia just mentioned, is A material which at room temperature can be stretched repeatedly to at least twice its original length and upon immediaterelease of the stress, will return with force to its approximate original length.

Not included are such products as the so-called snyt-hetic waxesf as for example, the ethylene telomers of Hanford and Joyce U. S. Patent 2,440,800, since these materials are generally products of low molecular Weight and in any event have a molecular weight well below 10,000.

The silica sols above described can advantageously be employed with phenolic resins such as phenolformaldehyde resins. They may aslo be used in terpene phenolic resins. The resins may be of the Novolak or of the infusible types. The phenolic resins may be A-stage resins, or resols; B-stage resins, or resitols; or resites, or C-stage resins. If the silica sol is added to a resin having a molecular weight below 10,000, however, the resin should be further polymerized until the molecular weight exceeds 10,000. The phenolic resins may be used in accordance with customary practices and may include fillers and modifying agents in addition to the silica.

The silica sols may be included similarly in resorcinolformaldehyde resins, urea-formaldehyde resins, phenolresorcinol-formaldehyde resins, and phenol-furfural resins. Similarly, the silica sols may be used in furan resins. The furan resins have'been particularly successful in the preparation of cold-setting cements, and the described silicas may be included in such adhesiva as fillers. They may, of course, be included in furan resins used as coatings, as impregnants, and as moldings and castings.

Melamine resins may also be modified by the inclusion of the described silica sols. Urea-modified melamines and co-condensation products of melamine-formaldehyde resins and diglycol carbamate may also be used.

The silica products described can also be included in the polyester resins such as the copolymers of diallyl phenyl phosphonate with monomers such as methyl methacrylate, vinyl acetate, diallyl phthalate, diallyl succinate, diallyl oxalate, and unsaturated alkyd resin mixtures. Other polyester resins may also be used.

The described silica sols may also be used to modify alkyd resins. These are prepared, of course, by polymerization of phthalic anhydride, maleic anhydride, or sebacic acid or other high molecular weight polybasic acid, with a polyhydric alcohol such as glycerol, the glycols, pentaerythritol, or sorbitol.

The acrylic resins may also be modified. These in- I clude polymeric methyl methacrylate and related polymers such as cyclohexyl methacrylate. The silica products are particularly advantageous in methyl methacrylate polymers because the transparency is not affected adversely when moderate quantities of the silica are used.

The silica products can also be included in vinyl polymers and copolymers. These include such products as polyvinyl chloride, polyvinyl acetate, polyvinyl-chloride acetate, vinyl, chloride-vinylidene chloride polymers, and polyvinyl acetals. In compounding, the vinyl resins. the silica can be used as heretofore described together with the fillers and plasticizers ordinarily employed. The value of the silica is particularly great in coated fabrics prepared using vinyl compounds.

Particularly good results are obtained by the inclusion of the described silica sols in polyethylenes. The prod uct may advantageously be included in either films or cast objects of polyethylene. A polytetrafluoroethylene may similarly be modified.

The described silica sols can. be used with advantage in styrene polymers and copolymers. The silica may be used, for instance, in polystyrene, in mixtures of polystyrene and chlorinated diphenyl, in copolymers of butadiene and styrene and in polymers of styrene-isobutylene and copolymers of maleic anhydride and styrene. The silicacan be used with advantage in styrene foam and in emulsions or solutions containing polystyrene and from which films are later formed.

The silica sols may similarly be used in nylon (polyamide) resins. The silica serves to add to the already great dimensional stability and thermal stability of nylon. They may be used to modify coumarone-indene and polyterpene resins. They can be used in casein and other protein plastics including casein-formaldehyde plastics.

As has been observed above, the. silica products previously described can be used in polymeric organic plastic materials generally and these include rubber and related elastomers. The elastomer may be any rubberlike polymeric material. The term elastorner is a general descriptive term for this class of products and may be regarded as an abbreviation for elastopolymer or elastic polymer. (See Advances in Colloid Science, volume 2, 1946, at page XXV.) As here used it includes the high molecular weight natural caoutchouc, as well as synthetic rubbers and rubber-like materials such as neoprene, butyl rubber, and the styrene-butadiene corolymer known as CR4.

More particularly, some of the elastomers included 12 are butadiene copolymerized in various ratios with styrene, butadiene copolymerized in various ratios with acrylonitrile, polymerized butadiene, polymerized, 2,3- dimethyl butadiene, polymerized2-chlorobutadiene, 1,3,- isobutylene copolymerized with isoprene, copolymers of butadiene and methylmethacrylate, butadiene copolymerized with methylvinyl ketone, and various other.

copolymers of butadiene with ethylenic-unsaturated hydrocarbons. .It will be noted that the invention is applicable to diene elastic polymers as a class.

The silica sols can be included in such modified rubber products as chlorinated rubber and cycle rubbers prepared by the action of chlorostannic acid, for instance, on natural or synthetic. rubbers. They may also be included in elastomers containing multiple sulfur linkages, such as the Thiokol types.

There is a group of organic. polymers, having properties somewhat different from those of the polymers just discussed, with which silica may advantageously be employed according to the present invention. This group includes naturally occurring polymers such asv water-insoluble starch, natural gum resins such as. Shel-1 lac, copal gum (including Manila gum), rosin, and ter-.

penes. It will be noted that in their lower degrees. of polymerization some of these organic polymers. are somewhat water-soluble, and it is intended to include in the scope of this invention only those which have a molecular weight in excess of 10,000 and are water-insoluble. The copal gum and shellac dispersions containing silica are particularly useful when they also include a wax,

such as carnauba wax or other ester wax, since the combination makes excellent floor waxes having pronounced.

skid resistance.

Resinous organic polymers which also have, valuable properties in floor coatings when dispersed together with silica are given in the following tabulation. If the molecular weight of the polymer is in excess of 10,000 the polymer may be used alone with the silica, but if the molecular weight is less than 10,000 the polymer may be used in combination with another polymer which has a molecular Weight above 10,000.

WATER DISPERSIB LE BESINS Specific Index of Melting Saponi- Acid Resin Gravity Refrac- Point, C. fication Number Chemical Type tion 1 Number X597" 8 N 150465 a 130-138 A modified moleic type. Amberol 750 l 1.20 5 155475 -115 Atortlfiedammonia-soluble maleic ester of rosin. Durez 15546 5 l. 17 1. 542 T 150:1:3 $:3

WAX-SOLUBLE RESINS Lewisol 28 1. 138 11 143 36 A maleic alkyd-modified ester of rosin. Piccolytc 8-85 12 0. 98-1. 0 B 85 0 4 0 4 A polyterpene predominantly beta pinene. Durex 219 13 1. 085 l. 552 7 50-60 A terpene phenolic. Pcntalyn A 1.08 1. 54 11 110 A pcntaerythritol ester of ro n- PentalynC 1. 09 l. 546 11 135 15 A modified pentaerythrital ester of rosin.

1 Determined by the conventional method at 25 C.

Determined by the mercury method.

a Product specifications listed on container.

4 Characterized in "Amberol 750 in Wax Emulsions," Product Bulletin, form 2011, published by Robin 1!: Haas 00., June 1950.

Determined by A. S. T. M. method D12730.

Characterized as "Duroz 15325 ground to 20 mesh, in Technical Data Bulletin of U. S. Industrial Chemical Co. dated August 16, 1950.

1 etermined by the capillary tube method.

l Characterized in Resinews," Vol. XV, published by U. S. Industrial Chemical 00., copyright 1947.

Viscosity =13, Gardner-Boldt at 25 C. on 60% solution (by weight) in toluene.

Characterized in "Hercules Synthetic Resins, published by Hercules Powder 00., copyright 1947.

ll Softening point, determined by Hercules drop method.

11 Molecular weight averages about 650. Ash less than 0.1%. Characterized Lu Piccolyte, the Versatile Resin, published by Pennsylvania Industrial Chem. Corp, copyright 1948.

Durez Protective Coating Resins in Emulsion-Paste and Liquid Waxes, published by Durez' Plastics and Chemicals, Inc.

. '13 Still other organic polymers which, when modified with silica according to this invention, make excellent films and effective floor coatings the slip-resistant wax-like type even without wax present are the following:

Pliolite Latex 150. Reference-Bulletin 1501, Chemical Products Division, Goodyear Tire & Rubber Company..

Resinous, copolymer hydrocarbon. (Pliolite latices are believed to be styrene butadiene copolymers, the styrene content increasing as the number increases.)

Total solids 30%.

Viscosity 30 cps.

pH 9.5-l0.5.

Specific gravity 1.01.

Surface tension 35-40 dynes/ cm.

Mechancal' stability No coagulation in 10 minutes under high speed agitation.

Storage stability Excellent.

Pliolite Latex 170. ReferenceBulletin 1701, Chemical Products Division, Goodyear Tire & Rubber Company. Resinous, copolymer hydrocarbon.

' Mechanical stability Excellent.

Reference: Butadiene-Styrene 'Resinous Copolymers, J. D. DIanni, L. D. Hess, W. C. Mast, Goodyear Tire and Rubber Co., Akron, Ohio. Ind. & Eng. Chem, vol. 43, p. 319 et seq., February 1950. P. 323 Weatherford and Knapp (3 7) discussed the properties and applications of a. latex (Pliolite Latex 190) containing a resinous copolymer of about'10%' butadiene and 90% styrene.

X-600 X-620 X-630 Total Solids 394l- 39-41 39-41. 4 Specific Gravity at 25 C"... 1.03.-. 1.04 1.05.

.Viscosity (centipoises at 25 C.) l30. 5-15 -25. pH 8-9.5- 8-9 9.5-11. Plasticizer. None.-- Santicizer Santicizer Plasticizer content, percent drys i s do 33 33.

Hycar Latices. Reference-Service Bulletin 47-H1, B. F. GoodrichChemical Company, October 1, 1947.

Stable suspensions of spherical particles of butadiene acrylonitrile type American rubber, 0.06 micron in diameter and negatively charged. These suspensions contained a fatty acid soap for stability. OR-25 is resistant to organic solvents when dried. Y

"Hycar OR-25 Total solids 40%.

Particle size 0.06 micron. I I Specific gravity 1.002 at 25/4 C. Viscosity 42 cps. at 25 C. Surface tension 51 dynes/cm. Color Buff.

Rhoplex-Rohm 8c Haas-Resinous Products Division Bulletin.

Rhoplex WN-75 is an aqueous dispersion of a nonionic, acrylic-type polymer. (Rhoplex WN- and FRN are similar.)

Solids (percent) 40. Colloidal Charge Neutral. Specific gravity (25 C./25 C.) 1.08. Viscosity, cp., 12 R. P. M 195.

Stable toward Acids as 10% H2804 Bases as NHs Saltsmany I Temp. 60C., 4 wks., 200 C., 5 min. Freezing, 1% coagulated Agitation-Good MAKING THE NOVEL COMPOSITIONS In preparing a product of this invention an aqueousdispersion may first be produced by separately preparing an aqueous dispersion of the organic polymer and mixing it with an alkali-stabilized colloidal silica solution or it may be prepared by dispersing the organic polymer directly in such alkali-stabilized colloidal silica solution. By the former method, for instance, one might simply mix an aqueous dispersion of polyvinyl acetate with an aqueous alkali-stabilized colloidal silica solution, whereas by the latter method one might disperse polyvinyl acetate in.

the alkali-stabilized colloidal silica solution in a suitable mixer such as a Banbury mixer. It will be understood that dispersion of the organic polymer may be facilitated by first softening it with plasticizers, dissolving it in organic solvents, or melting it prior to dispersion in the aqueous phase. Alternatively, plasticizers may be added to the dispersion after it has been formed.

When the organic polymer is an elastomer, the alkalistabilized colloidal silica may be added to a latex of the elastomer,- that is, to an aqueous dispersion of the elastomer. In this manner very uniform dispersion of the silica throughout the latex is achieved. The latex and the colloid silica may then be coagulated, resulting in the formation of a solid elastomer composition containing-the dispersed colloidal silica.

The choice of coagulant will, of course, depend in part upon the particular stabilizing agent originally usedv to stabilize the elastomer latex. Where the latex is stabilized under alkaline conditions with ammonia, the coagulating agent may be an acidic material such as calcium chloride. The coagulating agent may also act by abstracting the water from the mixture, and thus an organic solvent such as methanol or acetone may be used. In like manner mutual coagulation of the colloidal silica and latex may be accomplished by evaporating off the water or by freezing it, or by electrodepositing the elastomer particles in combination with the silica.

Dispersions of the organic polymer for mixing with the silica sol may be prepared by any of various ways with whichthe art is familiar. The polymer may be ground in an attrition mill, such as a ball mill or edge-runner mill, together with a dispersing agent and the liquid dispersing medium, such as water. If a particularly stable liquid dispersion is desired, the dispersion may be passed through a colloid mill or homogenizer, or other device which subjects the mixture to high shear.

To prepare directly a dispersion comprising the silica in a solid matrix of organic polymer, a number of methods are available. The polymer may be melted, the silica sol added, and the water of the sol driven off' by further heating. Similarly, the polymer may be subjected to intensive working, as ona rubber mill, a sigma-arm mixer, or an ink mill, and. the silica sol may be added to the mill during the working operation. Heat may be supplied to aid evaporation of the water added with the sol. Alternatively, the silica sol may be added to a monomer and the monomer may then be polymerized. The method of mixing will in any event be selected with a view to the character of the final product desired and to the nature of the polymer being modified.

CHARACTERISTICS OF PRODUCTS The products of this invention are characterized by being solid dispersions containing an alkali-stabilized colloidal silica and a water-insoluble organic polymer having a molecular weight of more than 10,000. The silica is in the form of substantially discrete and spherical particles which are dense and have an ultimate particle size of .01 to .03' microns. The polymer is a plastic and may be elastomeric.

The proportions of silica and polymer in the products may be widely varied. Two per cent by weight of silica inthe polymer-silica mixture provides an appreciable modification of the properties of the polymer, but amounts up to equal parts of silica and polymer may sometimes be used to advantage. ordinarily larger when the dispersion is a joint dispersion of the polymer and silica sol in a liquid medium.

USES OF THE NOVEL COMPOSITIONS Compositions of this invention are useful for a variety of purposes. For instance, the solidified films made by drying dispersions containing latex and alkali-stabilized colloidal silica are useful as adhesives. The adhesives preferably contain organic amines as stabilizers as set forth in U. S. application Serial No. 128,255, filed November 18, 1949, by Horning and Shay, now U. S. Patent 2,601,291. In foamed polychloroprene, inclusion of alkali-stabilized colloidal silica. permits replacement of polymer without loss of modulus and with no other undesirable change of properties. Dispersions of alkyd resins and of nitrocellulose in combination with alkali-stabilized colloidal silica can be dried to form solid products which are improved coating compositions. Phenolic resins containing alkali-stabilized silica. inaqueous dispersion can be dried tosolid films which'act as binding agents. Water paints of the aqueous emulsion type, inwhiclr heat-bodied oils and oil-modified alkyd resins are dispersed with water along with pigment, can be dried to films which show improved adhesion and durability when alkali-stabilized colloidal silica is added. Similarly, nitrocellulose emulsion coatings adhere better to smooth, hydrophilic surfaces when a minor proportion of sodium hydroxide-stabilized colloidal silica is present. Polyvinyl acetate emul sions containing the type of colloidal silica involved in" this invention dry to harder, more water resistant films because of the insolubilizing and hardening action of the colloidal silica. Further uses will become apparent from the examples.

- EXAMPLES The invention will be better understood by reference to.

ing about. byweight of solids there was: added 22."

The proportion of silica employed is parts of a 30% solution of. sodium hydroxide-stabilized colloidal silica. After thorough mixing, the emulsion was applied by a knife-type coater to a heavy cardboard in such thickness as to result, after drying, in a film. of polyvinyl acetate about 0.003 in thickness. The dried film was found to remain firm and to resist blushing when wetted with water, in comparison with a control film containing no silica, which readily softened and became tacky after soaking in water for 2 minutes. Also it was found that the coating containing silica was much rnor receptive to printing ink than the control.

Example II Colloidal silica of less than 0.03 micron particle size, when incorporated into latex as an alkalirstabi'lized colloidal solution, definitely improves the adhesion of synthetic rubber to cellulose fabrics such as rayon and to nylon fabric. For example, a latex formulation was made up as follows:

GR-S (styrene/butaldiene copolymer) #3 latex (60% solids) Zinc oxide 5 Phenyl beta-naphthylamine antioxidant 2 Sulfur 2 Mercaptobenzothiazole accelerator 1 Butyl zimate 0.5 Carbonblack Gastex l0 AquarexD surfacing agent 1.5 Sodium hydroxide 1.0 Colloidal solution of alkali-stabilized colloidal silica containing 30% by weight of S102, l0

An identical latex formulation was also prepared for purposes of comparison containing no silica. Woven rayon fabric was coated with approximately 15% by weight of these latex compositions, dried for three to five minutes at I00 0., and again coated and dried. The coated fabric was then placed in contact with a typical GR-S compoundeduncured synthetic rubber formulation and vulcanized under pressure for forty-five minutes, using a pressure of forty-five pounds steam. For testing, oneinch strips were cut out of the, assemblies. and the adhesion measured by pulling the fabric from the rubber to which it was vulcanized.

On a comparative basis, the fabric which had been pro-coated. with the latex formulation containing colloidal l Example III Sodium hydroxide-stabilized colloidal silica may advantageously be mixed with a dispersion of phenol-formaldehyde resin, either inthe (a) or the (b-) stage of. polymerization. For the formulation of adhesives, the (a) stage is preferred. For example, to 50 parts of an 18% Example IV To 5.0' parts of a concentrated water emulsion paint consisting. of a. dispersion of pigment and binder along with a dispersed heat-bodieddrying oil (linseed oil), sufiicient. water wasadded to give a paintable consistency. To another 50 parts of the concentrated paint, were added Parts by weight" Asbestos paper was about 100 parts ofa 30% solution of sodium hydroxide stabilized colloidal silica, giving a rather viscous fluid.

The thirty per cent product was made as described in the Bird patent by concentrationof efiluent. This was ldODfl by boiling the efiluent slowly in a pot. As thus prepared the sol must be used rather quickly for it is not at all stable towards gelling. The stability is satisfactoiy after mixing with the emulsion which dilutes the sol.

Equal volumes of each of the concentrated water emulsion paint mixtures above described, were mixed to give an aqueous paint dispersion which was still slightly too thick to apply with a brush, but which became workable When a small amount of additional water was added.

This aqueous paint mixture was brushed on a smooth paper alongside a control containing no silica. After airdrying for about one hour, the paint film containing silica had a much better resistance toward rubbing when wet, than the unmodified paint, and upon flexing the paper the coating showed much better adhesion, especially on areas which had been printed with an oil ink, prior to application of the paint. It was evident that the silica improves the adhesion of the water paint to the surface of the paper especially where the paper is oily or greasy with dirt and also improves the washability of the finish. lt

is believed that the silica provides an improved bond -.between the Water-insoluble dispersed oil binder and the other constituents of the paint film.

Other drying oils such as poppy and tung oils can be heat-bodied and used instead of the heat-bodied linseed oil of this invention.

Example V 100 parts of blown castor oil were emulsified into 100 parts of an 18% solution of sodium hydroxide-stabilized colloidal silica with the aid of 4 parts by weight of carboxy methyl cellulose as the dispersing agent. The resulting emulsion was coated as a thin film on a clean sheet of steel, permitted to dry in the air for 3 hours, and then baked for 1 hour at 130 C. The resulting film was definitely harder than a control similarly prepared without the inclusion of colloidal silica. It was also noted that the film containing the colloidal silica airdried'rnore rapidly than the control.

Example VI A compostion of this invention containing starch as the dispersed insoluble organic polymer was made up in the following proportions: v

, Parts by weight Starch (50 fluidity pearl) 16 Clay (kaolinite-type subcretaceous) 64 Water 110 Sodium-hydroxide-stabilized colloidal silica solution (18% SiOz) 10 The clay was first mixed with of the water and the colloidal silica solution was then added. The clay slip was then added with stirring to a starch suspension made by dispersing the starch in the remaining of the water.

This aqueous dispersion was applied as a coating to several diflerent types of paper, including bleached sulfite, rag and kraft, by both brushing it on, and by a doctor blade technique. After drying it was found that the clay was present on the papers as a uniform, adherent coating. This coating Was foundto be resistant to wet-rubbing, that is, the coating was not easily rubbed off by abrasion when wet; it resisted removal by soaking, being fast after soaking for an hour; it stiffened thin papers; and it took ink marking satisfactorily. A control coating, prepared in identical manner except that .no colloidal silica was present, was less water-resistant and did not take ink satisfactorily.

Example VII The remarkable effectiveness of alkali-stabilized colloidal silica as a reinforcing agent in the elastomer, poly- 2-chlorobutadiene 1,3 (hereafter referred to as polychloroprene) was demonstrated by making up the following latex formulation: l Parts. by weight Polychloroprene (5.0% aqueous dispersion using sodium resinate as dispersing agent, commer- The silica was incorporated into the mixture by stirring. Rubber films were prepared by the coagulating 'dip method, in which a mold was first dipped in a coagulating solution composed of 41.7% calcium chloride, 41.7% calcium nitrate, 8.3% acetone and 8.3% methanol. The dipped molds were permitted to stand in air until the organic solvents had evaporated, leaving a thin film of calcium salts on the surface. They were then dipped into the above compounded latex for 10 minutes, removed slowly and again dipped briefly into the coagulating solution, then washed for four hours with water at 50 0., dried four hours in air at 70 C., and cured in air for 45 minutes at C. The following physical tests on the film are shown in comparison with a control made in exactly the same way, except that no silica was used in the latex formulation:

Elastomer Property Oontain- Control ing Silica Modulus at 600% elongation lbs/sq. in 1, 775 860 ASTM Permanent Set percent; 8 10 Abrasion Test 1 002 004 Inches depth abraded ofi with a Taber abraser with a 1,000 gram weight and H-182 abrasive per 1,000 cycles.

Polychloroprene compositions in which a silica of particle size of about 0.05 micron and an extinction coeflicient of 0.67 was used as a reinforcing agent, had a modulus at 600% elongation of 1290 lbs/sq. in., and when a commercial carbon black Micronex was used the modulus was 1450 lbs./sq.'in.

Example VIII Outstanding advantages are achieved according to this invention by incorporating alkali-stabilized colloidal silica solutions into elastomer latices which are to be converted into foam sponge. To illustrate, a foamed sponge olychloroprene was prepared according to the following formulation:

Parts by weight Polychloroprene latex containing 60% polymer in aqueous dispersion. (commercially known as This formulated latex Was then whipped into a foam by beating air into it. Samples were vulcanized thirty minutes at the temperature .of steam atforty pounds pressure. A control sample was prepared in the samemanner .ex-

madam 19 cept that water was added in place of the solution of colloidal silica,'in order to rovide a tdrmulatien'identical with the above except that no siliea was present.

The load-carrying capacity of the resulting sponge containing silica, when measured by a compressiondeflection method, was 0.91 lbs/sq. 'in in comparison with 0.33 1b./sq. in. for the control. This increase in load-carr ing capacity can be otherwise obtained only by increasig the poly'ehldr'o ren'e content .per unit V61- ume by about 50%, or by using twenty parts by Weight of a filler such as clay, 'which, however, gives a sponge product which does "not recover as fully after compression as the material containing silica. The reinforcing action of the silica in comparison "tbthe rubber is especially sadvanta'geous since Zthe sponge retains its resilience and recovery from compression me much greater degree than when other fillers are used. The sponge has the additional advantage that "in spite of :its good load-carrying capacity, \it is much lighter in weight than .a poly- 'chloroprene sponge containing no silica "but su'fficient polychloroprene 'to give therequired load=carrlying capacity. This is especially ir'nportant :in :the 'case of .sponge rubber used for cushions in automobiles, airplanes, and

Example IX To demonstrate the effect of colloidal silica in improving the adhesion between polychloroprene "and rayon fabric .pre-coated with a polychloroprene 'late'x containing colloidal silica, acomposition was made 'upin a manner similar to that of Example II 'exe'e'p'tusing polychloroprene latex instead of GR-S latex for pre-coating the fabric. The pre-coated fabric was then vulcanized against a polychloroprene compound having the following composition:

Parts by weight The force required to strip the fabric in this base was 14.8 pounds for the sample containing silica, while it was only 6.8 pounds for the control containing no silica.

Example X This example illustrates the use er colloidal "silica in an aqueous "dispersion "of a s'tyrene butadie'n'e 'coploymer and the formation of films -therefrem.

A colloidal silica sol containing 30% SiOz, prepared by the process of Bechtold and Snyder and having the properties described above for the especially prefe'rred'sols of this process, was diluted with distilled water to a solids contentof -1-2%. Morpholine was added equal to 15% by weight-of the silica solids. An aqueous dispersion of Pliolite Latex 170 was diluted with distilled water to a-solids content of 12%. -Morpholine was added equal to 15% by weight of the polyme'r'solids.

A film cast from the aqueous dispersion of the P1iolite Latex 170 and allowed'to dry at room temperature, was continuous and transparent but had little body and was wrinkled and tacky.

One part of the silica sol was then mixed with four parts of the Pliolite dispersion, both being stabilized with morpholine as described, and alfilm was casti f'rom the homogeneous mixed dispersion, and -was dri'ed at room temperature. The film was stifier than that of the resin without silica, and had no tack. R was "similar in appearance to Vinylite sheeting.

A third film was cast on glass and dried at room temperature from a mixture er 2 parts of the m'or holine 2' 5) stabilized Pliolite dispersion and one part of the 12% silica sol not stabilized with morpholine. The film was hard and transparent and was not tacky. It was very adherent to glass and could not be stripped off as a film. Thus the presence of colloidal silica improves the properties of films prepared from the Pliolite latex.

Example XII 86 4O Hydrocarbon solvent 10 Zinc oxide -u 5 Antioxidant 2 A silica sol of the especially preferred Bechtold and Snyder type above-described, containing 30% SiOz was added to this adhesive. To separate portions of the ad hesive were added 5 and 10 parts by weight of silica, based on the solids weight of polychloroprene.

The adhesive strengths, in pounds per inch width, of bonds made with the unmodified and modified adhesives were as follows:

' 0 5 10 Parts Silica per Hundred Adhesive Strengths Leather to Leather 7. 5 12.0 21.5 Canvas to Canvas 5.0 10.0 17v 0 CHARACTERIZATION TECHNIQUES Particle size measurement by the electron microscope The size of the silica particles and the fact as to Whether or not they are non-agglomerated, that is, substantially discrete, can be directly observed if the silica is spread out in an extremely thin layer and examined with the electron microscope. Since the limit of resolution of the electron microscope is well below the 10 millimicron limit of the particles, there is no difiiculty in ascertaining Whether particles of the desired size are present. By examining the silhouette of the particles it is possible to measure the particle diameter in those cases where the particles are lying in such a way as to present more than a semicircle of cross section. Where less than a semicircle of cross section of a particle can be observed due to the fact that it is obscured by other particles, the particle is not measured. However, by counting and measuring a large number (several hundred) of particles which are lying in such positions as to present at least a semicircle of silhouette, the arithmetic mean particle diameter, i. e., the number average diameter, dn, the surface area average diameter, ds, and the particle size distribution can be determined. From the surface areaaverage diameter, a specific surface area, Sc, can be calculated. This is the specific surface area which the ultimate units would have if they were n0nagglomerated, discrete, dense particles, having an entirely exposed smooth-surface.

'In counting the particles, they are grouped according to diameter, as measured in millimicrons; each group consists -of particles lying within a five millimicron range, for example, 10-15 millimicrons, 15-20. millimicrons, 20-25 millimicrons, etc. The surface areaof the particles within each group is calculated from the average diameter of the group and the number or particles within the group. The total surface area of all the counted particles may th'enbe calculated by summation of the areas in each group. The specific surface area is this total surface 21 area, divided by the total mass of the particles counted, the latter being determined by summation, using the known density of the silica.

In carrying out this calculation, it is convenient to calculate first a surface area average diameter, ds, which would be the diameter of a spherical unit having the same specific surface area as that of the total collection of counted units.

This may be expressed mathematically, as follows:

6X 10 2720 'densityXd, d, The arithmetic mean particle diameter is simply the number average particle diameter, dn, Which is determined from the formula where m and air are defined as before and n is the total number of particles counted.

In determining the size of silica particles by the electron microscope it is necessary to avoid changes such as agglomeration or coalescence of the particles which may occur in drying the sols. This is minimized by diluting the sol and allowing the waterto evaporate at room temperature. The sample may be diluted to a concentration of from about 0.25 to 0.01% SiOz, but in general the maximum dilution will be used which permits retention of a sufiicient number of particles in the field to permit counting of the particles as described above.

The particle counts are made from electron micrographs taken at 5,000 diameters magnification enlarged to 25,000 diameters by conventional photographic printing techniques and finally projected on a screen to a total magnification of about 250,000 diameters. Methods of mounting the samples, and counting and measuring the particles are described by J. H. L. Watson in an article entitled Particle Size Determinations with Electron Microscopes in Analytical Chemistry, volume 20, pages 576584 for June 1948. The method of calculating particle size distribution is described in an article by L. R. Sperberg and H. M. Barton in Rubber Age, volume 63, pages 45--51 for April 1948.

Surface area determination by nitrogen adsorption The density of the silica particles may be determined by comparing their surface areas as calculated from electron micrographs as described above with the surface areas as determined by nitrogen adsorption. The method used for measuring specific surface areas by low temperature nitrogen adsorption is described in A New Method for Measuring the Surface Areas of Finely Divided Materials and for Determining the Size of Particles by P. H. Emmett in Symposium on New Methods for Particle Size Determination in the Sub-Sieve Range, p. 95, published by The American Society for Testing Materials, March 4, 1941. The value of 16.22 for the area covered by one surface adsorbed nitrogen molecule was used in calculating the specific surface areas.

When evaporating the sols to dryness for nitrogen adsorption determinations on the particles therein, the sols should be adjusted to a pH of about 3 to 4, and dried at room temperature and then at C. If the sol is evaporated in the basic pH range, the specific surface area as determined by nitrogen adsorption will be significantly less than if the sol is evaporated in the acidic pH range of 3-4. This decrease in specific surface area upon evaporation in the basic region appears to be due to a coalescence of the silica particles.

If the specific surface area as determined by nitrogen adsorption is not substantially greater, for example, not more than 25% greater, than the specific surface area as calculated from electron micrographs the particles are dense and the sol is suitable for use in the process of this invention. If the silica particles are porous they will be penetrated by nitrogen and the nitrogen adsorption Will be relatively high, and consequently the specific surface area by nitrogen adsorption will also be highmuch higher than would be expected on the basis of the direct observation of the particles by means of the electron microscope. On the other hand, if the specific surface area by nitrogen adsorption is not substantially greater than that calculated from electron micrographs, the particles are substantially smooth, discrete, non-porous spheres. This confirms direct observations as made with the electron microscope.

Measurement of relative viscosity Relative viscosity refers to the ratio of the viscosity of a solution or sol to the viscosity of the solvent. The viscosity is measured at 25 C. on a sol having a pH of 10 and containing 10% SiOz by weight. A sol which is electrolyte-free must be adjusted to pH 10 for the purpose of this measurement by the adition of sodium hydroxide. A sol containing less than 10% SiOz may be concentrated up to 10% SiOz by vacuum evaporation of water at room temperature and a more concentrated sol may be diluted back with water to 10% for measurement.

The viscosity measurements used in determining relative viscosity may be made according to conventional methods provided they are capable of adequate precision. Measurements made with a modified Ostwald capillary pipette for instance, under properly controlled conditions of temperature are adequate for the purpose.

The relative viscosity of the preferred sols used in this invention should be 1.15-1.55 measured at 10% SiOz and a pH of 10.0. The lower limit corresponds to the relative viscosity of a sol comprising as the dispersed phase only dense, discrete, spherical silica particles. This limit may also be supported by theoretically derived calculations. Sols having a relative viscosity greater than 1.55, on the other hand, may contain significant amounts of low molecular weight silica, particles of silica gel, or nonspheroidal particles, all of Which interfere with the concentration of the sols to high silica contents.

Measurement of specific conductance The specific conductance of the silica sols usedin this invention may be determined in accordance with conventional practices, such as those described by Glasstone, Textbook of Physical Chemistry, at page 874 et seq. The specific conductance is measured at 28 C. on a sol containing 10% by Weight of silica expressed as SiOz.

li Ieasuremeut of molecular weight by light scattering T e use of light scattering for the measurement of the absolute turbidity of silica sols (expressed as cmf and the weight average molecular weight of the silica particles in sols disclosed herein is described by Michael Sveda in Soap and Sanitary Chemicals July and August 1949, at page 121 et seq. Excellent results have been obtained, for example, using a B-S light scattering photometer manufactured by the Phoenix Precision Instrument 00., an improved form of the apparatus originally described 23 by R. Spe'i'ser 'andB. A. Brice in Journal 'ofthe Optical Society of America, volume 36, p.364, 1946. A light scattering photometer such as that described by P. P. Debye, Journal of Applied Physics, volume 17, May 1946, may also be used. The method of measurement is adapted from the method described byStei'n and De ty, Journal of the American Chemical Society, volume 68, p. 159, 1946.

- Measurement of turbidity Colloidal suspensions are more orles-s cloudy to visible light,depending upon the wavelengths, the finer the particles the clearer the solution. Other factors, such as the pH, salt content in the system, or the relative'refractive indices of the particles with respect to the solvent'or dispersion medium, of course, also play a role in determining the degree of turbidity, but, in general, the appearance of the solution gives some indication of the relative size of the dispersed particles in a given system, other factors being relatively fixed.

By comparing solutions of colloidal silica at the same concentration of SiOz, the relative transparency to light is an indication of the type of particles present. However, by working with monochromatic light and by measuring the percentage of light transmitted through a solution of a given depth at a given concentration of silica,

there can be calculated a constant, known as the extinction coefiicient."

By a combination of Beers and liamberts'law, the 1 following formula can be derived:

1' I l 3.. gm

where E{',,, =extainction coefficient =per cent by weight of SiOz in the colloidal solution. L=lengthof adsorption cell in centimeters.

:100, corresponding-to the intensity oflight of the shecified wavelength transmitted by the pure dispersion medium which in this case is water.

lzobserved intensity of light of the given wavelength transmitted by the sol as compared to the tnansmission of pure water.

It will be noted that the extinction coeflicient for a given type of solution is a constant which is independent of the concentration of the solution and of the length of the cell through which the light passes, but the numerical value depends upon the manner in which the concentration of the solution is expressed and upon the units in which the length of the cell is measured. in this case, the extinction coefiicient is given in terms of the percentage by weight of silica in solution and in terms of centimeters of cell length. in the near ultraviolet range having a wavelength of 400 millimicrons. A Beckman quartz spectrophotometer Model DU is used in making the measurements. The preferred sols which have been found to be particularly useful in this invention have an extinction coeificient of less than 0.25. The measurements are made at 25 C.

on a sol diluted to 1% SiOz and at apH of 910.

The measurements are made using light Z4 Chemical analysis Standard analytical techniques are used throughout with the-following limitation.

Deteriizfmztion 0f titratable alkalinity.'-In order to de- I tel-mine the Si022M20 ratio, it is necessary to analyze the solution for silica and titratable alkalinity. The latter is determined by titrating the sol with HCl to a pH of 4.5 I

using aglass electrode pH meter.

This application is a division of'my copending United States application Serial No. 239,853 filed August 1, i, as a continuation-in-part of my then co-pending United States application Serial No. 699,086 filed September 24..

1946, now abandoned. and of-my then co-pending United States application Serial No. 172,021 filed July 3, 1950, now Patent2,597,872, the latter being a continuation-inart of my United States applicationSerial No. 699,087 file September 24, 1946, now abandoned. l

l claim:

1. A composition comprising a solid matrix of a water insoluble organic elastomer foam having a molecular weightlgreater than 10,000, selected from the group consisting of natural rubber, polychloroprene', chlorinated rubber. and copolymers of conjugated dienes in which there has been dispersed an alkali-stabilized. colloidal silica having an average ultimate particle size of about 0.01 to 0.03 micron, the alkali being substantially all outsideof the silica-particles.

2. A composition comprising a solid matrix of a'waterinsoluble organic conjugated diene elastomer foam having a molecular weight greater than 10,000 in which there has been dispersed an alkali-stahilized colloidal silica having an average ultimate particle size of about 0.01 to 0.03 micron, the alkali being substantially all'outside of the silica particles.

' 3. A composition comprising a solid matrix of a waterinsoluble organic elastomer foam havingv a molecular weight'greater than 10,000, selected from the group consisting of natural rubber, polyehloroprene, chlorinated rubber, and .copolymersof conjugated dienes, in which there has been dispersed an alkali-stabilized colloidal silica having an average ultimate particle size of 0.015 to 0.03 micron and an SiOztalkali oxide mole ratio of from 10:1 to 500:1 and consisting of dense, amorphous. non-agglomerated particles, the alkali being substantially all outside of the silica particles.

4. A composition comprising a solid matrix of a waterinsoluble organic elastomer foam having a molecular weight greater than 10,000, selected from the group consisting of natural rubber, polychloroprene, chlorinated rubber, and copolymers of conjugated dienes, in which there has been dispersed an alkali-stabilized colloidal silica having an average ultimate particle size of 0.001 to 0.100 micron and anSiOzzalkali oxide mole ratio of from 10:1 to 500:1 and consisting of-dense, amorphous, hy drated silica particles, the alkali being substantially all Outside of the silica particles.

References Cited in the file of this patent UNITED STATES PATENTS 2,597,872 Iler May 27, 1952 

1. A COMPOSITION COMPRISING A SOLID MATRIX OF A WATERINSOLUBLE ORGANIC ELASTOMER FOAM HAVING A MOLECULAR WEIGHT GREATER THAN 10,000, SELECTED FROM THE GROUP CONSISTING OF NATURAL RUBBER, POLYCHLOROPRENE, CHLORINATED RUBBER, AND COPOLYMERS OF CONJUGATED DIENES IN WHICH THERE HAS BEEN DISPERSED AN ALKALI-STABILIZED COLLOIDAL SILICIA HAVING AN AVERAGE ULTIMATE PARTICLE SIZE OF ABOUT 0.01 TO 0.03 MICRON, THE ALKALI BEING SUBSTANTIALLY ALL OUTSIDE OF THE SILICA PARTICLES. 